This invention relates to a remote-control system for controlling revolution of a motor provided for a sewing machine.
As having been well known in the art, a remote-controller is detachably connected to an electronic sewing machine and step-operated by the sewing machine operator to control revolution of a motor and therefore to control an operating speed of the sewing machine.
The prior art remote-control system can be classified into two types. One is of a two-wire system in which a variable resistor [VR] in a remote-controller is connected by a couple of wires to the sewing machine, as shown in FIG. 1 (b). An output voltage [Vout] from the variable resistor [VR] is determined by the following equation (1): ##EQU1##
The r1=0 when no step-operation is applied to the remote-controller and is varied in proportion to a stepping amount of the remote-controller. Accordingly, the output voltage [Vout] will draw a quadratic curve as shown in FIG. 1(a). This means that the output voltage [Vout] will not increase in proportion to the stepping amount of the remote-controller, resulting in less controllability.
Due to errors in resistance values of the resistors VR and R1, the output voltage [Vout] would be varied within a certain range. More particularly, in FIG. 1(a), a curve referred to by [VRmin] shows the output voltage [Vout] when the actual resistance values of the resistors VR and R1 provide the minimum VR to R1 ratio, whereas another curve [VR max] traces a value of the output voltage [Vout] in the case of the maximum VR to R1 ratio. A reference value [Vtyp] of the output voltage [Vout] exists in the middle of [VRmax] and [VRmin].
Now, provided that the remote-controller is made operative or turned on when the operator steps the same to the extent that 12.5% of the output volume [Vout] is obtained. Such starting point of the remote-controller will be determined in dependence upon the output volume [Vout] but may actually be varied in a wide range from [ONmin] to [ONmax]. The operator could not exactly know when the sewing machine actually starts to operate while stepping the remote-controller by degree.
The operation of the prior art remote-controller of two-wire system has been uncontrollable as above described and therefore not put into practice, while having in turn an advantage that the sewing machine can be operated with safety. If one of the wires connected to the variable resistor VR should be broken down, Vout becomes zero resulting in an emergency stop of the sewing machine.
Another type prior art remote-controller is of three-wire system as shown in FIG. 2(a) and (b). With this improved remote-controller, the output volume [Vout] is obtained in accordance with the following equation (2): ##EQU2##
As the resistance r2 increases from zero in proportion with the stepping amount of the controller, the output volume [Vout] increases on the straight from zero as apparently shown in the above equation (2) and also in FIG. 2(b). Thus, the controller of this type has improved controllability. FIG. 2(b) also shows that the controller is turned on in response to a substantially constant stepping amount, which is effected at 12.5% of the output volume [Vout] in the example shown, even when the resistors VR and R1 have incorrect resistance values.
According to this prior art controller having improved controllability and responsibility, however, when one (terminal No. 3, for example) of the wires connected thereto is broken down, Vout will rapidly increase to an extremely high level just the same as Vcc so that the sewing machine will be rotated at a tremendous speed. It has therefore been required to provide means for detecting disconnection of the wires, as a safety device. A single cord containing three wires would be of relatively a large diameter, which could not easily be reeled in.